Process of and apparatus for the recovery of helium from a natural gas stream

ABSTRACT

A process for recovery of helium from a natural gas stream containing components such as acid gases, moisture, hydrocarbons, nitrogen and helium. The process includes steps of feed gas preparation, refrigeration cooling and liquefaction, and separation of helium from the hydrocarbons and then nitrogen. Apparatus for carrying out the process is disclosed.

United States Patent [1 1 Fan [ June 26, 1973 PROCESS OF AND APPARATUSFOR THE RECOVERY OF HELIUM FROM A NATURAL GAS STREAM [75] Inventor: SinChou Fan, East Lansing, Mich.

[731 Assignee: Commonwealth Associates Inc.,

Jackson, Mich.

22 Filed: Sept. 18,1970 211 Appl.No.:73 ,300

s21 U.S.Cl ..62/29,62/22,62/17, 62/30,62/23 [51] lnt.Cl. F25j 1/00, F25j3/00, F25j 3/02 [58] Field oISearch 62/22, 23, 24, 27, 62/28, 29

[56] References Cited UNITED STATES PATENTS 3,057,168 10/1962Becker..... 62/29 3,148,966 9/1964 Kitchen.... 3,359,744 12/1967 Bolez62/23 3,355,902 12/1967 Crawford 62/28 2,765,637 10/1956 Etienne 62/273,293,863 12/1966 Karbosky.. 62/23 3,512,363 5/1970 Harper 62/273,037,359 6/1962 Knapp 62/22 OTHER PUBLICATIONS Garwin, Leo; A look atHelium Hydrocarbon Processing, April 1969 pgs. 97-100 PrimaryExaminer-Norman Yudkoff Assistant Examiner-Arthur F. PurcellAttorney-Olsen and Stephenson [57] ABSTRACT A process for recovery ofhelium from a natural gas stream containing components such as acidgases, moisture, hydrocarbons, nitrogen and helium. The process includessteps of feed gas preparation, refrigeration cooling and liquefaction,and separation of helium from the hydrocarbons and then nitrogen.Apparatus for carrying out the process is disclosed.

3 Claims, 1 Drawing Figure NVzl nOHQ NIS HOLNHANI PATENTEB JUN 26 I975 Q& Q

k INVENTOR SIN CHOU FAN NUQWOWWQ ATTORNEYS PROCESS OF AND APPARATUS FORTHE RECOVERY OF HELIUM FROM A NATURAL GAS STREAM BACKGROUND OF THEINVENTION The present invention relates to the recovery of helium fromnatural gas streams.

Natural gas is the commercial source of helium. The helium contents varyfrom a few parts per million to several percent. Presently natural gasstreams having a helium content of approximately 0.4 percent and up arebeing processed for helium. In the future, the helium processors,however, may have to extract helium from gas streams with very lowhelium contents so that more effective procedures for helium recoverywill be required than are now employed. For example, gas with a heliumcontent of 0.05-0.07 percent is being processed in Russia now.

Natural gas is a complex gas mixture and contains variable quantity ofmethane and heavier hydrocarbons. There are also present some inertgases such as nitrogen and acid gases such as CO and H 8. In addition,there are added during production and transmission impurities which canbe anything from dusty solids to lube oil. All the heavy hydrocarbons,inert and acid gases will affect the process of liquefaction to someextent and must be reduced to a tolerable level. Also, all the addedimpurities should be removed completely from the gas streams.

SUMMARY OF THE INVENTION The present invention is directed to a heliumrecovery plant wherein the feed or natural gas is taken from a highpressure pipe line and the tail gas is recompressed back to the samepipe line. The extraction process for the helium recovery can be dividedinto the steps of feed gas preparation, cooling and partial liquefaction, and the separation of the helium from hydrocarbons andnitrogen.

One of the improvements of this process is the operation at a higherpressure level, as compared to the existing processes, which will resultin refrigeration savings. Another is taking the advantage of a doubledistillation column for the separation of nitrogen and helium from otherhydrocarbons. With the use of such a double column, the upper section isoperated at a lower pressure and the lower section at a higher pressure,which carries the major separation. By so doing, the refrigerationrequirement for reflux condensation in the lower section can be properlymet by utilizing part of the bottoms in the upper section. This willresult in additional savings in refrigeration. The bottom stream fromthe upper section will be used as purge gas for effective regenerationof the desiccant dryers.

The bottom stream from the lower section will be first utilized as arefrigerant to cool the incoming gas and then compressed back to thepipe line. The higher the pressure level of this stream, the less thepower requirement will be. This will result in savings in fixed andoperation costs.

According to one form of the present invention, a process is providedfor the separation of helium from a natural gas stream containing acidgases, moisture, hydrocarbons, nitrogen and helium comprising the stepsof purifying the atream by removing acid gases and moisture at arelatively high pressure of about 700 psia, gradually cooling the gasstream to a temperature sufficient to partially liquefy the purifiedstream to provide a liquid portion of relatively heavier hydrocarbonsand a gaseous portion at about 600 psia and 126" F enriched in helium,nitrogen and relatively lighter hydrocarbons, further cooling theenriched gaseous portion to about 130 F and expanding the same into thelower section of a double distillation column at a pressure of about 500psia to produce a lower liquid stream and a top stream, expanding thetop stream to a pressure of 300 psia into the upper section of the samecolumn in heat transfer relation above said first lower section toproduce a top stream containing mainly, nitrogen and helium and a lowerliquid stream in the bottom of the upper section in cooling relation tothe top stream of the lower section, cooling the top stream in saidupper section to about 255 F and then expanding the top stream ofnitrogen and helium into a succeeding distillation column at a pressureof about psia to produce a lower liquid stream of mainly nitrogen atabout 290 F and an upper stream of crude helium, and passing said lowerliquid stream of nitrogen in heat transfer relationship to the topstream in said upper section of the double distillation column forcooling the top stream to 255 F. The lower liquid stream from the lowersection of the double column represents high pressure tail gas at about500 psia which can be passed in heat transfer relation to said feed gasstream for cooling the same and which is then vaporized and recompressedto about 700 psia for discharge into the source of the natural gasstream. The lower liquid stream from the upper section of the doublecolumn is passed in heat transfer relation to the gas stream for coolingthe same and is then utilized as purge gas for the regeneration of thedesiccant dryers.

Thus, operating pressures of about 600 psia are utilized initially inseparating the gaseous portion en riched in helium, nitrogen andrelatively lighter hydrocarbons, and thereafter, pressures of about 500psia are utilized in the lower section of the double column. By virtueof these operating procedures, a less amount of power is required inorder to recompress the tail gas and return it to the original source orstream of natural gas. Also, the effective use that is made of the tailgases for cooling purposes reduces the extent of refrigeration requiredduring cooling and liquefaction.

The process is carried out by apparatus which is constructed andarranged to permit recovery of the helium through a series of steps atthe pressures and temperatures set forth. In part, this apparatusincludes unique double column separation unit for separating thenitrogen and helium from the gas stream.

Accordingly, it is among the objects of the present invention to provideimproved method and apparatus for extracting helium from a natural gasstream characterized by the effective and economical results realized.

More specifically, the primary objects of this invention is to providean improved process for effectively recovering helium from natural gasesand to accomplish this recovery with a minimum requirement of externalpower for compression and refrigeration. An additional object is torecover helium with a method which minimizes the pressure differencesbetween the feed gas and the tail gas.

Other objects of this invention will appear in the following descriptionand appended claims, reference being had to the accompanying drawingforming a part of this specification.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates schematicallyone embodiment of apparatus for carrying out the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before explaining the presentinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction andarrangement of parts illustrated in the accompanying drawing, since theinvention is capable of other embodiments and of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation.

Referring now to the drawing and to the follwing Table I, the inventionwill be described in greater detail. It should be pointed out that theinformation listed in Table I is only one example. In case thecomposition of the feed gas shown on Column A of Table I varies, theresults in other columns will vary also. In Table I, the data set forthin the columns under the letters A-U, inclusive, correspond respectivelyto locations in the drawing identified with these letters.

The feed gas is taken from the pipe line through the conduit 12 to a gasfilter 14 where impurities such as dust solids, lube oil, and the likewill be filtered out and can be removed at 16 and the purified gas flowsthrough the conduit 18 to the absorber 20.

Carbon dioxide and water vapor are the most troublesome impurities dueto freezing characteristics at low temperature. The ice formation on thesurfaces of heat exchanger fins and tubes will not only add resistanceto heat transfer, but also in the extreme case, block the gas flow.Systems for carbon dioxide removal are known. These include aminetreating, hot potasium carbonate scrubbing, water washing, etc. Theabsorber 20 is part of an amine treating system for removal of carbondioxide. As shown in column A of Table I, the natural gas stream flowingthrough the conduit 12 contains carbon dioxide, which can readily beremoved in the absorber 20 together with any other acid gases which maybe present. As shown in the drawing, the gas umn. The gas from which theacid gases have been removed leaves the absorber 20 at the top via theconduit 24 with the gas composition being that shown in column B ofTable I.

the MEA solution. The regenerated MEA solution flows out of the stripper32 through conduit 34 and via the heat exchanger 30 to the amine pump 36for circulation back to the absorber 20 by way of the conduit 22. Forregeneration purposes, the stripper is provided with steam coils 37 atthe bottom and cooling coils 38 at the top. The stripped gases aredischarged from the top of the stripper through conduit 40. Any make-upMEA solution will be added into the system through the conduit 41.

The natural gas stream next flows through the heat exchanger 42 wherethe temperature of the stream is reduced to about F. Due to the loweringin temperature, there will be water condensation. The moisture contentin the gas stream is thus reduced, about 30 percent lower than theoriginal, and the condensed water being collected and removed in theseparator 44. The natural gas stream then flows through the conduit 46to the two desiccant dryers 48 and 50 which are piped in parallel asshown, one being in service while the other is in regeneration. Suchdryers are filled with desiccant materials such as molecular sieves. Thegas to be dried has the composition shown in column C of Table I andenters into the appropriate dryer at the top and flows out the bottomthrough the conduit 52.

In special circumstances the procedure of operation ofthe apparatusdescribed above may vary. For instance, the removal of acid gases anddrying may be carried out simultaneously in columns 48 and 50 if thecontent of such acid gases in the feed gas is very low. That is, thefeed gas will be lead directly to columns 48 and 50 and the columns 20and 32 and their auxiliaries will be eliminated from use.

The desiccant dryers, when saturated with moisture, are subjected toregeneration. The valves are so arranged and the regeneration may becarried out in the following three steps: (a) heating of the dryer toapproximately 600 F. by means of the gas heater 54 which is arranged toheat return low pressure tail gas flowing in conduit 56 to thistemperature; (b) stripping with the return low pressure tail gas fromthe conduit 56 for discharge through the conduit 58; and (c) by means ofthe return tail gas, cooling to the operating temperature ofapproximately 80 F. The low pressure tail gas flowing from the conduit56 and either through the gas heater 54 and one of the dryers 48, 50 forheating the same or directly through such dryer for cooling the samethen is discharged via conduit 58 to the power plant of the system. Thefuel gas for the heater 54 may be obtained from the conduit 58, asshown.

The purified natural gas stream having the composition shown in Column Dnow flows via the conduit 52 'lAllLIC l. MA'II'IIHAL liAliANtllC A" 15*l) l' I (i II" I .l l\' l M N (l I Q It S I ll ll('. 0. 4 0. 4 0. 4 0. 40. l 0. -l 0. 4 0. 4 'llnt'fl 0. 30 0. 0l 0. 3R 0. 0| N2. 3. 4 3. 4 I. 4I. 4 3. l 3. 'l 3. -l 3. 0 0. -l h 0. .5 0. 2 2. ti (,()7 3.5 Nll lI S(1H K0. 8 XII. X 80. X 80. X 80. H X5. X5 85. H5 20. Ii 0."). 25 0. 320. 3 0. 3 0. 05 (.glIn 0.5 0.5 0.5 0.5 0.5 0.!Jl 0.0l 0.0l 5.5!! Calla,2.5 ".5 2.5 2.5 2.5 2.5 (lnlllll. 0. l 0.4 0. 1 0. 4 0. 4 0.1 Moles/hiI02. 5 I00 I00 I00 00. 50 21 lit 50 3. 10 20. 5| 0. 5H 3.01 l). -'l-'lllqlA 700 (300 500 500 300 300 300 75 75 75 000 200 .500 F I00 100 80-35 -l00 l2ti I30 I50 -25?) l00 -3l l 200 15 Dry lmsis.

" Partially liquiliml state.

Operating comlililms given are :tpplnxinmli'.

The absorber 20 has an outlet 26 at the bottom through which a richsolution of the acid gases flows first through a liquid filter 28 andthen via the amine heat exchanger 30 to the stripper 32 for regeneratingto heat exchanger 60 equipped with a propane refrigeration system 62. Asshown in Column E, the exit temperature is 35 F at conduit 64. The heatexchanger 60 is a four stream design, one for the gas stream to becooled, one for the propane refrigeration, one for high pressure tailgas which will flow through the heat exchanger for cooling purposes fromthe conduit 66 to the conduit 68, and one for low pressure tail gas,also for cooling purposes, which will enter from the conduit 70 and willbe discharged through the conduit 56.

The main gas stream is further cooled to -100 F by passing through heatexchanger 74 and being discharged to the conduit 76. The heat exchanger74 is also a four stream design; one for the main gas stream, the secondfor the stream from the ethylene refrigeration system 78, the third forthe high pressure tail gas entering from the conduit 80 and dischargingvia the conduit 66, and the fourth for the low pressure tail gas bonseparation column 86 where the heavier hydrocarbons will be condensedand separated from the gas stream. The gas and liquid streamcompositions from the separation column 86 are shown in Columns G and S,respectively, of Table I.

Approximately 90 percent of the heavier hydrocarbons are removed fromthe feed gas in separation column 86. The heavier hydrocarbons from thebottom will be removed at 88 where they can be further processed intoethane and propane-plus, if this should be necessary. This can beaccomplished in the separation column 90 wherein ethane can be removedat the top via the conduit 92 and propane-plus can be removed at thebottom via the conduit 94.

In the separation column 86 cooling is provided at the top of theseparation column with the coils 96 by vaporizing part of the returnliquid via conduit 98. Heat is provided by the higher temperature gasvia conduit 99 at the bottom of the separation column.

The gas stream from the top of the separation column 86 contains mainlymethane, nitrogen and helium, and this gas mixture is further cooled toa temperature of approximately 130 F in the heat exchanger and vaporizer100. The latter is a four stream design; one is for the gas streamflowing through the conduit 102 to the outlet conduit 104, one is forthe high pressure liquid flowing through the conduit 98 vaporized in thevaporizer 100 and discharging through the conduit 80, and the third isfor the low pressure tail gas flowing from the conduit 106 anddischarging through the conduit 82, and the fourth is for the ethylenerefrigeration system 78. The partially liquefied gas stream in conduit104 next flows to the separation or fractional distillation column 108which is divided into two sections, the lower section 110 being operatedat a pressure of 500 psia, and the upper section 112 being operated at apressure of 300 psia. The conduit 104 passes through the lower portionof the lower section 110 to provide the necessary heat requirement inthe reboiler and then through the expansion valve 114 whereby the gas isexpanded to a pressure of 500 psia and discharged into an intermediateportion of the lower section as shown. The top streams from the lowersection 110-will be fed to the upper section via the conduits 115 and116 through the expansion valves 117 and 118 so that the pressure isreduced to 300 psia in the upper section 112. The feed to the topsection is designed to be approximately 25 percent of the feed gas tothe plant in the disclosed form of the invention. The gas stream or lowpressure tail gas discharged from the lower portion of the upper section112 will be used later for purging of columns 48 or 50 and then as plantfuel. The liquid stream from the bottom of the lower section will havethe composition shown in Column K of Table I, and the averagecomposition of the top streams will have the composition shown in ColumnJ of the table. Likewise, the vapor taken from the bottom of the uppersection will have the composition shown in Column N of the Table I. Aspreviously explained, the liquid from the bottom of the lower section110 passes back through the heat exchangers via the conduit 98 forcooling purposes, and vaporized, and there it is recompressed to 700psia by the compressor 111 and is discharged into the supply pipe line10.

The gas stream from the top of the upper section 112 contains mainlynitrogen and helium as is shown in Column M of Table I. This gas whichis at a pressure of 300 psia and a temperature of about 250 F next flowsto the helium separation or fractional distillation column 119 via theconduit 120 and after passing through the expansion valve 122 isdischarged into the column at a pressure of 75 psia. The temperature atthe top of this column is 314 F and at the bottom is 290 F. In order toachieve the temperature at the top, it is necessary that liquid nitrogenrefrigeration be used at 124. The liquid nitrogen refrigeration systemis conventional. The helium product which is extracted via the conduit126 has the composition shown in Column Q. This crude product can becompressed and stored in gas form in conventional storage bottles. Theliquid nitrogen is extracted at the bottom of the separation column 119and has the composition shown in Column R. This liquid nitrogen can bepassed via the conduit 128 through the cooling coils at the top of thesection 112 of column 108 to provide the cooling requirement for refluxcondensation. Additional liquid nitrogen will be added via conduit 127,if necessary.

From the foregoing description it will be recognized that the disclosedhelium recovery plant is designed to operate so that the pressure dropthrough the system is kept small. By so doing, there are savings inrefrigeration as well as in power which is required to recompress thetail gas to the inlet pressure. Thus, the process is more economical byvirtue of the operating pressures for the separation of the lighthydrocarbons, nitrogen and helium from the gas stream being maintainedat a high level. Also, maximum refrigeration economy is realized by theflow circuits disclosed. Of special significance in this respect is theconstruction and arrangement of the separation column 108.

I claim:

I. A process for the separation of helium from a natural gas streamcontaining carbon dioxide, moisture, relatively heavier and lighterhydrocarbons, nitrogen and helium comprising the steps of purifying saidstream by removing carbon dioxide and moisture from said gas stream at arelatively high pressure of about 700 psia, gradually cooling thepurified gas stream to a temperature sufficient to partially liquefy thepurified stream to produce a liquid portion of relatively heavierhydrocarbons and a gaseous portion at about 600 psia and 1 26 F enrichedin helium, nitrogen and relatively lighter hydrocarbons, further coolingthe enriched gaseous portion to about l F and expanding the same into alower section of a double section, fractional distillation column at apressure of about 500 psia to produce a lower liquid stream and a topstream, expanding the top stream to a pressure of about 300 psia in theupper section of the fractional distillation column in heat transferrelation above said lower section to produce a top stream of nitrogenand helium and a lower liquid stream in the bottom of the upper sectionin cooling relation to the top stream of the lower section, the topstream of said lower section being condensed and used as reflux in saidlower section, cooling the top stream in said upper section to about 255F and then expanding the top stream of nitrogen and helium into asucceeding fractional distillation column at a pressure of about 75 psiato produce a lower liquid stream of nitrogen at about 290 F and an upperstream of crude helium, and passing said lower liquid stream of nitrogenin heat transfer relation to the top stream in said upper section forcooling the top stream to 255 F.

2. The process that is defined in claim 1, wherein the lower liquidstream from said lower section is passed in' heat transfer relation tosaid gas stream for cooling the same and is then recompressed as a gasto 700 psia for discharge into the source of the natural gas stream.

3. The process that is defined in claim 1, wherein the lower liquidstream from said upper section is passed in heat transfer relation tosaid incoming gas stream for cooling the same and is then utilized aspurge gas in conjunction with the removal of moisture from the naturalgas stream and thereafter as plant fuel gas.

2. The process that is defined in claim 1, wherein the lower liquidstream from said lower section is passed in heat transfer relation tosaid gas stream for cooling the same and is then recompressed as a gasto 700 psia for discharge into the source of the natural gas stream. 3.The process that is defined in claim 1, wherein the lower liquid streamfrom said upper section is passed in heat transfer relation to saidincoming gas stream for cooling the same and is then utilized as purgegas in conjunction with the removal of moisture from the natural gasstream and thereafter as plant fuel gas.